AGU journal highlights — June 14, 2010

June 15, 2010

The following highlights summarize research papers that have been recently published or are “in press” (accepted, but not yet published) in Water Resources Research (WRR) and Geophysical Research Letters (GRL).

In this release:

How might prolonged drought impact California?

First images of Saturn lightning

Ozone recovery plus climate change may boost smog

Thickness of Arctic’s old sea ice steady since 2007

Midmantle plume detected below southern Africa

Weekend weather cycle: anthropogenic and natural

Cities face warming double whammy

False sunshine trend explained by nonuniform data

Uranium map illuminates Moon’s formation

Ionosphere affected by lower atmosphere variations

Imaging magma beneath New Zealand

Vegetation shapes drainage topography

Pacific Antarctic Ridge marks mantle discontinuity

Anyone may read the scientific abstract for any of these papers by clicking on the link provided at the end of each Highlight. You can also read the abstract by going to http://www.agu.org/pubs/search_options.shtml and inserting into the search engine the full doi (digital object identifier), e.g. 10.1029/2008WR007681. The doi is found at the end of each Highlight below.

Journalists and public information officers (PIOs) at educational or scientific institutions, who are registered with AGU, also may download papers cited in this release by clicking on the links below. Instructions for members of the news media, PIOs, and the public for downloading or ordering the full text of any research paper summarized below are available at http://www.agu.org/news/press/papers.shtml .Please note that papers not yet published (i.e. “in press”) are available only to journalists and public information officers.

1. How might prolonged drought impact California?

Recent studies show that prolonged episodes of extreme drought have occurred in the past and may be more likely under human-induced greenhouse warming. This is of particular concern to communities in California, which combined serve as the agricultural center of the United States. In this context, Harou et al. seek to answer an important question: Can current infrastructure systems adapt to possible future prolonged and severe drought?

Rather than sift through results from the myriad of climate models in existence, the authors instead turn to evidence of drought in geological and biological records. These records show that two extreme droughts, each 120� years long, occurred in California during the past 1,200 years. Average annual streamflows for these events were 40󈞨 percent of the average streamflows from the past few decades. Armed with these data, the authors use a model to simulate a 72-year drought with streamflow at 50 percent of current average rates. This simulated drought was fed into a hydroeconomic optimization model of water management, which covers the entire water supply and demand system of California and simulates how California could respond through using water-trading markets.

Results provide best-case estimates of economic costs and effects on water operations and demands. Most important, they show that California’s water supply system is flexible enough to respond to severe stress — adaptation strategies suggested for sustained drought are expensive but not catastrophic to the overall economy. Drought would, however, impose severe burdens on the agricultural sector and natural environment, which may be catastrophic to ecosystems and local communities. Nonetheless, the authors suggest that improving response flexibility through water-trading markets or other forms of water reallocation will prove vital to managing resources during future prolonged and severe droughts.

Title: Economic consequences of optimized water management for a prolonged, severe drought in California

Lightning, common on Earth, has also been detected on Jupiter, and previous studies have reported indirect evidence for lightning on Saturn. Visible lightning has now been imaged on Saturn for the first time. Dyudina et al. report that on 17 August 2009, the Cassini spacecraft detected visible lightning flashes on Saturn. It had been difficult to get images of lightning because Saturn’s nights are very bright due to its highly reflective rings shining in the sky, which make it hard to visually distinguish lightning. The recent equinox, during which most of the rings were in shadow, made it possible to image lightning flashes.

The researchers report that the lightning flashes are consistent with a single cloud flashing once per minute, and the visible energy of a single flash is comparable to that on Earth and Jupiter. They also note that the storm that produced the visible lightning had been active for nine months, much longer than storms on Earth or Jupiter. The study could help scientists better understand the formation and characteristics of lightning on Saturn and other planets.

Although ozone in the stratosphere (~10󈞞 kilometers, or 6󈞋 miles, in altitude) helps protect life on Earth from harmful solar ultraviolet radiation, at the lower altitudes in the troposphere, close to Earth’s surface, ozone is a major constituent of smog and has detrimental health effects. The stratospheric ozone layer had been depleted but recently has started recovering due to efforts to limit emissions of ozone-depleting chemicals.

Previous studies showed that climate change could lead to higher levels of ozone in the troposphere because it changes atmospheric circulation and accelerates the exchange of gases between the stratosphere and the troposphere. Stratospheric ozone recovery will probably further increase tropospheric ozone levels, according to a study by Zeng et al.

The researchers use a chemistry climate model to simulate the effect of stratospheric ozone recovery between 2000 and 2100 on tropospheric ozone levels. The authors find that, combined with higher levels of ozone in the stratosphere due to reduced emissions of ozone-depleting chemicals, climate change will result in greater levels of tropospheric ozone than previously thought, especially in the Southern Hemisphere during winter months. The results suggest that climate change and ozone recovery contribute equally to this effect, which could influence air quality and atmospheric chemistry.

Title: Impact of stratospheric ozone recovery on tropospheric ozone and its budget

Arctic sea ice extent has declined over the past several decades and in 2007 reached a record-breaking low. Since then, ice has begun to recover. Sea ice thinning is expected to continue with climate change, although there is significant natural variability in ice extent and thickness. To understand the effects of climate change on sea ice and to make forecasts, scientists need reliable sea ice thickness data, but Arctic sea ice thickness information is scarce. Some ice thickness data have been obtained from submarines in limited areas and from airborne and satellite radar altimetry, which suffer from uncertainties.

To contribute to assessment of the recent state of Arctic sea ice and to complement other observations, Haas et al. conducted a large-scale airborne survey of Arctic sea ice thickness in April 2009. For the first time, the technique of electromagnetic induction sounding was applied from a long-range fixed-wing aircraft, enabling the coverage of all key regions of old ice (ice that formed more than a year ago) in the time frame of one month.

The researchers compare their results with previous studies and find that although the areal extent of ice may have declined, the thickness of old ice has not changed much since 2007, and has remained within the expected range of natural variability.

Mantle plumes, columns of very hot material in Earth’s mantle, drive volcanic activity and transfer heat from Earth’s core to the upper mantle. A new midmantle plume has now been detected below southern Africa. Sun et al. analyze data from multiple events recorded by a seismic array in the Kaapvaal region of southern Africa. They find a narrow (less than about 150 kilometers, or 93 miles, in diameter) plume-like feature starting in the lower mantle at the top of the African large low-velocity structure, a region where mantle composition changes and seismic waves slow down. The results provide a detailed look at the plume and could help scientists understand the characteristics and dynamics of structures in the lower mantle as well as the geology of the southern African region.

It has been suggested that human activity causes a “weekend effect” in which meteorological variables show weekly cycles. For instance, researchers have observed that the difference between daily maximum and minimum temperatures (diurnal temperature range) increases during the weekends. Precipitation, wind, and cloud cover have also been shown to have weekly cycles in some locations. However, some studies have suggested that weekly changes in atmospheric conditions may have natural, not anthropogenic, causes. Kim et al. analyze temperature data covering more than 50 years to separate the natural and anthropogenic weekend effects. They find that Rossby waves, which are large-scale natural oscillations in the atmosphere, induce a natural weekend effect much stronger than the anthropogenic weekend effect. After removing the natural weekend effect, their analysis reveals a more accurate pattern of the anthropogenic weekend increases in diurnal temperature range. This shows that natural weekend effects should be removed before the nature of anthropogenic weekend effects can be clearly understood.

The urban heat island effect, in which an urban area is notably warmer than surrounding nonurbanized areas, will likely increase in coming decades, as urbanization is projected to increase dramatically, reaching 6 billion urban dwellers worldwide by 2050. However, urbanization trends have not been included in many climate model projections. To study how climate change and urbanization will interact and influence future urban microclimates, McCarthy et al. use an urban land model coupled with a global climate model to quantify the effects of climate change on the urban environment. They separate the effects of climate change from those of urbanization and find that in some regions, the increases in temperatures and extreme heat events due to urbanization could be as large as the effect of doubled atmospheric carbon dioxide. In addition, they find that climate change could magnify the urban heat island effect in some locations, increasing the temperature difference between rural and urban areas.

With climate change an increasingly important scientific and political issue, it is essential to have an accurate picture of the causes of regional climate changes. In most cases, warming has been related to a combination of anthropogenic and natural factors. However, regional data are often nonuniform and can be difficult to analyze, sometimes leading to incorrect conclusions.

For instance, a recent paper (N. Lockart et al., On the recent warming in the Murray-Darling Basin: Land surface interactions misunderstood, Geophys. Res. Lett., 36, L24405, 2009GL040598, 2009) suggested that a trend toward increasing sunshine hours over the Murray-Darling Basin in Australia accounted for an observed increase in temperature there. However, a new paper challenges that conclusion. Cai et al. note that the previous paper used data from 15 stations that have records of varying length and uneven geographical distribution over time. The early period of data included more stations farther away from the equator, while the later period included more stations closer to the equator. Since the analysis was performed during the winter season (when stations closer to the equator receive more sunshine), and since the climatological average was not removed from the station data, a spurious trend in sunshine hours was created by connecting the station records together over time.

The new paper shows that when these factors are accounted for, there is no trend of increasing sunshine. In addition, since the previous paper did not provide an explanation for the spurious increase in sunshine hours, a statement of causality was not possible in any case. Therefore, the authors conclude, the observed increase in temperature cannot be attributed to increased sunshine hours.

Title: Comment on “On the recent warming in the Murray-Darling Basin: Land surface interactions misunderstood” by Lockart et al.

Studies of abundances and distribution of radioactive elements can help answer questions about the formation and evolution of the Moon. Using data from instruments on board the Japanese Kaguya spacecraft (also known as SELENE), Yamashita et al. map the global distribution of uranium and the uranium-thorium ratio on the surface of the Moon. Uranium and thorium are radioactive elements that emit unique gamma ray signatures, which can be identified by Kaguya’s gamma ray spectrometer.

They find significant variations in the uranium-thorium ratio in different areas on the far side of the Moon, which had not been reported in any previous observations. This new observation calls into question some models of lunar formation that assume the primary lunar crust is uniform. The study will contribute to improving scientists’ understanding of the Moon’s composition and evolution.

Variations in the ionosphere, the charged upper portion of Earth’s atmosphere (about 100� kilometers, or 62� miles, in altitude), can affect radio communications and navigation systems. Solar activity is known to strongly influence the ionosphere, and recently scientists have recognized that lower levels of the atmosphere can also affect ionospheric conditions. But, these effects have been hard to see because the ionosphere varies so rapidly in response to solar and geomagnetic activity.

To explore connections between the stratosphere (which is about10󈞞 km, or 6󈞋 miles, in altitude) and the ionosphere, Goncharenko et al. analyze a sudden stratospheric warming event that occurred in the high-latitude stratosphere in January 2009, when solar activity was low. They find that a few days after that event, anomalous variations occurred in the total electron content of the low-latitude ionosphere. They explain that sudden warming of the stratosphere results from strengthened atmospheric planetary waves, which are topographically forced. These waves also drive atmospheric tides, which transfer energy upward to the ionosphere.

The researchers also ran a simulation of the effect, capturing characteristics similar to those they saw in the observed data, thus confirming that the ionospheric changes were induced by the planetary waves and tides. The results support other studies that have shown a link between the lower atmosphere and the ionosphere and could help improve forecasts of ionospheric weather conditions.

The Taupo Volcanic Zone, on the North Island of New Zealand, is an active region of intense volcanism with high heat flow. The high heat flux from this zone is comparable to that found in Iceland and at Yellowstone. It has been suggested that large volumes of magma in the Taupo Volcanic Zone come from a “mush zone” that contains a mixture of solid crystals and liquid melt. However, the structure of the underlying magmatic system driving the volcanism in the region is not well known. To learn more, Heise et al. use an electrical resistivity study to create a three-dimensional image of the magma distribution. Liquid melt is electrically conductive, whereas solid crystals are not. Their image shows a plume-like structure of high conductivity that they interpret as a zone of interconnected melt. The study could help scientists better understand the processes that drive heat flux and volcanism in the Taupo Volcanic Zone.

Drainage density, a measure of how much the landscape is dissected by channels, is known to vary among different climates. The prevailing thought holds that drainage density is low in arid regions because of low runoff; it increases in semiarid regions as runoff increases, and then decreases in subhumid regions as expanding vegetation counteracts the erosive runoff. Extremely humid regions may have high drainage density because of variable and torrential precipitation regimes. It is also commonly accepted that the differences in drainage density among climates are dependent on vegetation types. However, the steepness of the channels and the height of the peaks also influence drainage density, while themselves also varying with climate. Thus, teasing out the interrelationships among drainage density, vegetation type and cover, channel slope, and catchment elevations is a difficult task.

Collins and Bras tackle this challenge with a numerical model of landscape evolution that brings together geomorphology, hydrology, and ecology. Focusing on sediment-mantled landscapes devoid of landsliding, the authors ran a series of numerical experiments to see how rainfall regimes affect topography, particularly drainage density. They find that drainage density dropped from arid to semiarid climates, and then rose again toward subhumid, as is seen in observational data. The inverse is observed for mean elevation. Having conducted the experiments with a numerical model, however, the authors are able to trace the cause of the climatic dip in drainage density to its biophysical roots: the hydrological behavior of plants. That the model can reproduce a suite of topographic observations by omitting many features of nature for clarity’s sake — such as diverse plants and seasonality — suggests that what appear to be complex topographic patterns may be understood with relatively simple conceptual explanations.

Authors: D. B. G. Collins: Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts; now at National Institute of Water and Atmospheric Research, Christchurch, New Zealand; R. L. Bras: Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts; now at Department of Civil and Environmental Engineering, University of California, Irvine, California, USA.

Understanding the geochemistry along ocean ridges can help scientists understand the dynamics and evolution of the mantle. To complete sampling of the Pacific Antarctic Ridge, Hamelin et al. collected data on major elements, trace elements, and strontium (Sr), neodymium (Nd), and hafnium (Hf) isotopes in basalts collected along the ridge from 41 degrees to 53 degrees south. These were the first Sr-Nd-Hf isotope data collected in that area, a section of the Pacific Antarctic Ridge that does not have hot spots, which would affect the geochemistry.

The researchers find that samples are generally geochemically homogenous and typical of mid-ocean ridge basalt chemistry, though there are tight, coherent geochemical variations within the mantle. They looked at samples from both sides of the Menard transform fault, which crosses the Pacific Antarctic Ridge at about 50 degrees south. The transform fault is a major geological feature; the question has been whether it is also a geochemical feature separating two mantle domains, each with its own history. The researchers find that samples from different sides of the transform fault have slightly different isotopic composition, confirming that the transform fault is a discontinuity between two mantle domains with different geochemical characteristics.

Title: Sr-Nd-Hf isotopes along the Pacific Antarctic Ridge from 41 to 53°S